1. Field of the Invention
The present invention relates to a disc brake caliper for use in automotive vehicles and in particular to a new construction of such a caliper.
Disc brake calipers are well known in the automotive industry. Indeed most vehicles manufactured at least in recent years include disc braking arrangements which employ disc brake calipers. While disc brake calipers of various manufacturers will vary in relation to specific aspects of their calipers, the calipers generally have a similar overall shape, because they are mostly required to fit within the same overall type of structure. That is, almost universally, the caliper is required to fit within a wheel cavity, and about a rotor that rotates with the wheel. Thus, the overall shapes of calipers in general are, to a large extent, relatively similar and are dictated by the space available within the wheel cavity for positioning of the caliper.
2. Description of the Related Art
FIG. 1 shows the general shape of a prior art caliper 10, and that caliper includes a housing 11 and an anchor bracket 12. The housing 11 is movable relative to the anchor bracket 12 during brake actuation. In the FIG. 1 caliper the housing 10 includes three fingers 13 which depend substantially perpendicular from a bridge section 14 thereof, that in turn depends from a piston mounting section 15. The piston mounting section 15 extends in a plane substantially parallel to the plane of the fingers 13. The piston mounting section 15 includes a pair of elongate mounting shafts on which the anchor bracket 12 is mounted for relative movement in the direction A. While the mounting shafts cannot be seen in FIG. 1 due to their accommodation within the housing 11 and the anchor bracket 12, the shafts extend coaxially with openings formed in the arms 16 of the piston mounting section 15 and are fixed thereto by a head and nut arrangement 17, 18. That arrangement is only shown on one side of the caliper 10, but an identical arrangement at the other end 19 of the mounting section 15 is provided for the other of the two mounting shafts.
The disc brake caliper 10 is provided with a pair of brake pads on which a friction lining is mounted and those pads are forced into engagement with the disc rotor when a braking force is applied. Operation of the caliper 10 in this manner would be well understood by a person skilled in the art and is therefore not important for the purposes of the present invention. Instead, it is the particular construction of the caliper which is important and which will be the subject of the following description.
The housing 11 and the anchor bracket 12 of the brake caliper 10 are normally cast from metal. Casting the caliper housing and anchor bracket is appropriate from a manufacturing point of view, but in some cases the castings are tumbled or shot blasted after solidification to remove surface imperfections . In so doing a residual compressive stress is introduced into the surface which is the subject of that treatment, which advantageously can provide increased resistance to fatigue failure. However, subsequent machining of the cast surface which has been treated in the above described manner, has the effect of removing the metal imparted with residual compressive stress and that has the undesirable effect of reducing the fatigue strength of the casting in the machined area. For the most part, the surface finish which can be obtained by casting is satisfactory for the use to which the caliper is put. However, some surfaces require a more precise surface finish and in particular, the surfaces against which the brake pads are mounted require a smoother, flatter surface finish than is possible through normal casting techniques and therefore, those surfaces are normally machined. FIGS. 2 and 3 show by shading, the surfaces of the caliper 10 of FIG. 1 that would normally be machined for mounting of the pair of brake pads.
In FIG. 2, it can be seen that the machining takes place in the junction of the caliper 10 between the fingers 13 and the bridge 14, which is a section of the caliper formed as an arc, that is known to applicant as the xe2x80x9cinner finger radiusxe2x80x9d and that terminology will be used to describe that section of the caliper throughout this specification. Before machining, the inner finger radius is cast as a raised bead 20 between the inner surfaces of the bridge 14 and the fingers 13, which is then machined for mounting of the brake pad. The bead 20 is machined to form an arcuate surface extending from adjacent one edge 21 of the housing 11 to adjacent an opposite edge 22. The machined surface is shaded in FIG. 2 and is identified by the reference numeral 23. The machined surface 23 is interrupted midway between the respective edges 21 and 22 by a gap 24, which is provided for location of a brake pad thereagainst.
FIG. 3 illustrates the same housing 11 shown in FIGS. 1 and 2, but at a different angle to show the inside surface of the piston mounting section 15. Like FIG. 2, the machined surfaces are identified by shading.
A disadvantage with calipers manufactured to the above construction, is that machining of the bead formed in the inner finger radius results in an increased propensity for the caliper to fail in that area, because machining of that kind causes a localised reduction in the fatigue strength of the caliper as described earlier. That is, localised reduction of fatigue strength occurs due to removal by machining of residual compressive stress in the surface of the casting and also by the introduction of irregularities, such as scratches and notches that occur in the machined surface during the machining process. The fatigue strength of the caliper in the machined region can be recovered to some extent by work hardening of the surface, such as by shot peening, but that adds considerably to the cost of the caliper manufacture. It has been found that fatigue failure in this type of caliper 10, most often occurs in the machined section of the inner finger radius at either end 21 or 22 thereof.
Calipers are also required to be designed for maximum stiffness, as any deflection of the caliper during braking, reduces the effectiveness of the caliper performing that function. Stiffness can be increased by increasing the thickness of the caliper in those sections that are most likely to deflect, but the maximum thickness of a caliper is governed by the need to fit the caliper within the confined wheel cavity of the and the preference to maintain the rotor at as large a diameter as possible, for maximum braking efficiency. Thus the thickness of the caliper is required to be minimised as far as possible.
It is an object of the invention to provide a disc brake caliper which has a reduced propensity for fatigue failure. It is a further object of the invention to provide a disc brake caliper which has a reduced propensity for fatigue failure in the inner finger radius. It is still a further preferred object of the invention to provide a disc brake caliper that has improved stiffness.
According to the present invention there is provided a disc brake caliper having a cast metal housing and an anchor bracket which are connected for relative movement during brake actuation, said housing having a bridge section for bridging in use, a disc brake rotor and first and second axially spaced brake pad mounting faces extending substantially perpendicular to said bridge section for disposal in use, on either side of a disc brake rotor and for axial mounting of brake pads thereagainst, first and second inner finger radiuses extending respectively at the junction between said bridge section and each of said first and second brake pad mounting faces for radially supporting a portion of the periphery of a said brake pad, piston actuating means being provided for axially displacing a said brake pad away from said first brake pad mounting face into engagement with a disc brake rotor, each of said first and second brake pad mounting faces and said inner finger radiuses being machined over the surfaces of contact with the brake pads, said second inner finger radius between said bridge section and said second brake pad mounting face including raised sections at each end thereof relative to the section extending therebetween.
In a preferred arrangement, the inner finger radius includes a pair of unmachined sections extending inwardly circumferentially from either end of the inner finger radius and at least one machined section extending between the unmachined sections.
The use of the terms xe2x80x9cmachinedxe2x80x9d and xe2x80x9cunmachinedxe2x80x9d is used to identify sections of the caliper that are surface finish treated and untreated respectively, following the casting process used to form the relevant part of the caliper. Thus, the use of the term xe2x80x9cmachinedxe2x80x9d in relation to a certain part of the caliper, will identify that part as having been surface treated, such as by milling, grinding and/or polishing, to remove metal therefrom. A part of the caliper that is defined as being xe2x80x9cunmachinedxe2x80x9d will not have been surface treated in this manner, although it may have been subject to other treatments, such as shot peening , shot blasting , surface hardening, heat tempering etc.
A disc brake caliper according to the invention advantageously has increased resistance to fatigue failure in the regions in which prior art calipers are most prone to fail, ie at each circumferential end of the inner finger radius. Additionally, in the raised sections of the inner finger radius, the caliper has a greater cross-sectional thickness and in those sections, the strength and stiffness of the caliper is greater than in the reduced section. Thus, the overall strength and stiffness of the caliper is also increased. These benefits are however, realised without necessitating large changes to the shape or configuration of the caliper, or to the shape of the brake pads, and a caliper according to the invention is not envisaged to require substantial manufacturing changes to those traditionally employed.
In a preferred embodiment of the invention, the inner finger radius is cast so that the raised sections extend axially inwardly, toward the piston mounting section of the caliper, so as to increase the cross-sectional thickness of the caliper in those sections and thus further increase the strength and stiffness of the caliper. The axial extent of the raised sections preferably should not exceed the thickness of the brake pad backing plate which is mounted against the fingers, otherwise in use, the rotor may come into contact with those sections when the friction lining attached to the pad is fully worn.
The invention has application to calipers employing any number of piston actuators. The prior art caliper illustrated in FIGS. 1 to 3 includes two piston actuators which would be mounted in cylinders 25 and 26 respectively (see FIG. 3) and the invention has been developed principally in relation to this type of caliper. However, the invention equally has application to calipers having a single piston actuator, or calipers having three or more actuators.
With respect to a caliper having two piston actuators, in a preferred embodiment of the invention, the inner finger radius includes a further raised section substantially intermediate the ends of the inner finger radius. That section may also have an inward axial extent equal to approximately the thickness of the brake pad fitted thereto. The existence of a third such section further enhances the strength and stiffness of the brake caliper in the region of the inner finger radius.
In a further preferred embodiment of the invention, the section of the caliper provided for mounting a brake pad opposite the pad mounted against the caliper fingers, also includes a pair of raised sections extending circumferentially inwardly to a reduced section extending therebetween. Moreover, where the caliper includes provision for two or more piston actuators, a reduced section may be disposed intermediate each piston actuator. The raised sections of this part of the caliper advantageously can be extended axially inwardly toward the fingers of the caliper a greater distance than the complimentary sections of the inner finger radius, given that actuation of the caliper causes movement of the brake pad mounted against the piston mounting section, away from that section.